The present invention provides an improved method and apparatus for conditioning the gases discharged from a cupola that is used to process material, such as in iron melting, in order to decrease the cost of controlling air pollution.
In processes and apparatus of the prior art, as in U.S. Pat. No. 3,518,817, water spray towers have been used in combination with secondary gas conditioning means such as a gas scrubber and separator stack. In such devices, the secondary conditioning apparatus are generally designed to accommodate the condition of the gases discharged from the cupola, usually referred to as "off-gas", during the "burn-down" period of operation. The burn-down period is understood to mean that period of time near the end of the processing of material when the addition of charge to the cupola is discontinued. It is during this period that the heat of the gas which is rising through the bed of material, usually referred to as "bed-gas", is no longer used to preheat the charge of metal, coke and flux. The extra heat therefore is discharged through the cupola stack at temperatures approaching 2700.degree.F. This additional heat increases the size and cost of equipment which is necessary to comply with pollution control regulations. For example, the discharge duct which conveys the off-gas from the top of the cupola stack to a subsequent dust control apparatus such as a spray tower may be refractory lined at temperatures up to 1800.degree.F with silica-based refractories. If the temperature in the duct were permitted to exceed 1800.degree.F heavier, more expensive alumina-based refractories would be necessary.
The alumina-based refractories are nearly twice as heavy as the silica, resulting in turn with the necessity of increased structural costs for support. Additionally, the alumina-based refractories are less resistant to abrasion than silica-based refractories, so that maintenance costs are also increased by the use of alumina-based refractories.
Therefore, in an effort to reduce the temperature of the bed-gas during burn-down to normal operating temperatures of below 1800.degree.F at the stack, apparatus of the prior art have added large quantities of dilution air through the charge door. The past design criteria therefore has been to accommodate the additional quantity of dilution air, which may be an additional 20% to 25% of the normal off-gas, and to design the capacity of the gas discharge ducts and dust collection system according to the increased quantity of gas. In other words, the prior art has found it necessary, as part of their gas conditioning systems, to provide ducts and dust collector systems for the burn-down period which are up to 25% larger than the ducts and systems which are necessary during normal operation of the cupola.